A lot of Emergency equipment must operate even in power failuer situations. This is why telephony systems typically include battery-backed power at the central office or other location on the service provider side of the network. For example, a battery or other power source is coupled in parallel with the power provided from a line voltage.
In recent years, telephony systems have been developed that provide wireless local loops. In these systems, the subscriber equipment includes a radio transmitter connected to the subscriber's telephone and mounted on the subscriber's premises; typically, on an external wall or roof. With a wireless local loop, telephone service is provided to remote subscribers with no physical connection to the telephony network and thus no means for providing power to the subscriber's equipment. A power circuit coupled to a line voltage at the subscriber's premises typically provides power to the equipment. To reduce the chance that the subscriber will lose service due to a loss of line voltage, battery back-up is typically provided for the power circuit. This is also true with respect to solar powered equipment which is typically equiped with battery backup.
Several issues have evolved in the design of power circuits related to providing the battery back up at the subscriber's premises. The first issue is referred to as "deep discharge." When power is lost from the line voltage, the battery provides power to the load, e.g., the telephone, transmitter, and other equipment. As a consequence, the battery discharges over time, i.e., the voltage on the battery reduces with time. If the line voltage is lost for a substantial period of time, the battery voltage may drop significantly. This can damage the battery, e.g., lead acid and nickel cadmium batteries can go "dead" completely. Some power circuits include a low voltage detection circuit that disconnects the battery when the voltage falls below a threshold.
A further issue relates to providing current to charge the battery. The power circuit provides both current to the load and current to charge the battery. If a single power supply is used to provide both the load current and the charging current for the battery, too much current may be provided to the battery. This occurs when the load draws a small portion of its maximum current. In this case, the battery may be severely damaged by the excessive current. Typically, this problem is addressed in power circuits by using two separate power sources. A first power source provides power to the load with high power and constant voltage and a second power source provides power to charge the battery with low power and a constant current. Unfortunately, such power circuits are expensive, complex and consume a large amount of space at the subscriber's premises. Further, these power circuits provide a heavy power drain.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an arrangement for providing power to a telecommunications circuit with reduced cost and reduced risk of damage to battery-backed systems.